Growth hormone (GH) may directly affect the expression of some liver cytochromes P-450 inducible by xenobiotics, but this has been difficult to establish with pituitary ablation in living animals. Therefore, we incubated adult rat hepatocytes on a laminin-rich matrix, matrigel, a new system that permits hepatocytes to respond to xenobiotics with induction of the microsomal hemoproteins, the cytochromes P-450, in culture as they do in the intact liver (Schuetz, E. G., Li, D., Omiecinski, C. J., Muller-Eberhard, U., Kleinman, H. K., Elswick, B., and Guzelian, P. S. (1988) J. Cell. Physiol. 134, 309-323). Indeed, when cultures were treated with phenobarbital, there was a rise in mRNAs for the cytochromes P-450b/e and P-450p accompanied by a rise in mRNA for 5-aminolevulinate synthase, the rate-limiting enzyme in heme biosynthesis. Analysis of nuclei from the matrigel cultures established that phenobarbital treatment had activated transcription of the P-450b/e gene. Co-incubation of the cultures with physiologic concentrations of growth hormone completely blocked the induction of these P-450 mRNAs and partially blocked the rise in 5-aminolevulinate synthase mRNA. Induction of P-450p by isosafrole also was inhibited strongly by GH, whereas P-450p induction by pregnenolone 16 alpha-carbonitrile or dexamethasone was affected only weakly by GH. The effect of GH was specific inasmuch as phenobarbital-inducible expression of P-450 reductase, glucocorticoid-inducible expression of tyrosine aminotransferase, and basal expression of albumin were unaffected by the presence or absence of growth hormone. Nuclear analysis revealed that growth hormone inhibited phenobarbital-induced P-450b/e gene transcription, whereas the hormone was without effect on transcription of the liver-specific gene, tyrosine aminotransferase. In contrast, the addition of another peptide hormone, insulin-like growth factor I, was without inhibitory effect on P-450 gene expression. We conclude that growth hormone acts specifically and selectively in direct contact with the hepatocyte to control xenobiotic induction of some liver drug-metabolizing enzymes.